There is presently no known efficient and accurate device or method for the continuous, real-time monitoring of volatile organic compounds in fluid streams, even though such pollutants have received much attention recently and many VOCs are toxic or carcinogenic even in trace amounts and lead to ozone formation in the troposphere. Examples of common volatile organic pollutants include aliphatic and aromatic hydrocarbons such as hexane and benzene; chlorinated organics such as vinyl chloride, dichloro ethane; oxygenated compounds such as methanol, methylethylketone and also nitrogen or sulfur containing compounds. However, the VOCs are usually present in low concentrations (sub ppm.sub.v and even sub ppb.sub.v levels) and it is difficult to accurately monitor them on a continuous basis.
Without the means for continuous monitoring, it is the current practice to utilize whole air samplers such as tedlar bags and canisters (e.g. EPA method TO14) or sorbent cartridges. With whole air samplers, several liters of a sample are collected and taken to a laboratory for analysis.
With the sorbent based methods, the sample is passed through a cartridge containing one or more adsorbents such as Tenax, XAD-2 and charcoal where the VOCs are trapped (EPA Method T01, EPA Method 5). Thereafter, the analytes are thermally desorbed or solvent extracted for analysis. Measurement of low concentrations is achieved by concentrating the analyte from a large volume of sample, with such measurements being made at off-site laboratory facilities.
While such methods are generally effective and accurate, the samples are normally transported to the laboratory for analysis by gas chromatography (GC) with a suitable detector. Accordingly, they cannot be utilized for continuous on-line analysis to provide information on a real-time basis as required for effective pollution control and for meeting regulatory requirements.
An important feature of any continuous, on line GC analysis is the means by which the sample is introduced to the analytical column. The injection band should be sharp (usually less than a second wide) to provide good GC resolution. Multi-port sample valves are used as injectors in continuous GC monitoring analysis. The valves use a sample loop for injection with typical injection volumes ranging from a few microliters to 1-2 milliliters for capillary columns. These valves automatically make injections from a sample stream, on an intermittent basis, to the GC column. However these valves have definite limitations, foremost of which is that the injected sample size is only between a few microliters to at most a few milliliters. Small samples produce a small signal and lower the sensitivity and increase detection limits. Detection becomes a problem particularly with sample streams that have sub parts per million or per billion concentration levels. Although desirable, the injection of larger sample quantities from large loops causes excessive band broadening and degradation of chromatographic resolution. Sample valves are therefore inadequate for accurate environmental monitoring of VOC concentrations.